Nonlinear optical material, process of production of same, and nonlinear optical device and directional coupling type optical switch using same

ABSTRACT

A nonlinear optical material composed of a polyimide obtained from a diamine and/or a diacid anhydride or dithioacid anhydride substituted by a portion having a nonlinear optical effect or of molecules having a benzocyclobutene structure substituted at the portions having an nonlinear linear optical effect.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonlinear optical material, a processfor production of the same, and a nonlinear optical device and adirectional coupling type optical switch using the same.

2. Description of the Related Art

In optical data processing equipment for optical exchanges, opticalcomputers, optical interconnections, and the like, optical switcheswhich exchange the light among waveguides by electrical signals areindispensable. As the basic form of an optical switch, the directionalcoupler shown in FIG. 1 for example is known. When two waveguides 1 and1' are made to approach each other to about a wavelength of light 2,transfer of optical power occurs between the waveguides at a certaincoupling length. A directional coupling type optical switch is one whichcontrols the transfer of the optical power by causing the index ofrefraction of the coupling portion to change by, for example, theelectro-optic effect.

The conventional directional coupler had been comprised between twowaveguides on a plane, but we newly proposed a multilayer typedirectional coupler obtained by superposing layers of waveguides andconstituting directional couplers between those layers (see JapanesePatent Application No. 4-48961). Using this multilayer directionalcoupler, it becomes possible to achieve a high degree of integration ofthe optical circuits.

The distance over which the transfer of light occurs (i.e., couplinglength) is determined by the thickness and index of refraction of thecore and cladding layers of the waveguides. When fabricating adirectional coupler, it is necessary to bring the waveguides close toeach other by exactly the distance suitable for the coupling length. Ina conventional planar type directional coupler, it had been easy tobring the waveguides into close proximity by exactly the necessarydistance, but a suitable technique for a multilayer type directionalcoupler as we had newly proposed had not been conventionally known.

At the time of fabricating the directional coupler, it is necessary tobring the two waveguides 1 and 1' close together as shown in, forexample, FIG. 1, by exactly the interval suitable for the couplinglength. In the conventional planar type directional coupler, it had beeneasy to bring the waveguides close to each other by exactly thenecessary interval, but in the multilayer type directional coupler wenewly proposed, there had been no suitable technique for this.

In the case of constituting a directional coupler, the coupling constant(κ) between waveguides and the difference (β) between the propagationconstants (δ) of the two waveguides become important (for example,Yariv. Introduction to Optical Electronics, 3rd edition, published inJapan as Hikari Erekutoronikusu no Kiso, 3rd edition (Maruzen), Chapter13). The length (i.e., coupling length) of a directional coupler isexpressed as π/2κ, while the maximum value of the transfer of opticalpower at the time of insertion of light into a waveguide is expressed byκ² /(κ² +δ²). That is, for use as a directional coupler, it is necessarythat δ be sufficiently larger than δ.

The conventional optical coupler had been fabricated by formation ofwaveguides on a substrate of an electro-optic material such as LiNbO₃by, for example, diffusion of Ti (for example, see Nishihara, Haruna,and Suhara, Optical Integrated Circuits (Ohm Co.), Chapter 10). At thistime, while the electrode structures differ depending on the opticalaxes of the crystal, basically in the state with no electric fieldapplied, the propagation constants are equal (δ=0). In two waveguidesbetween which transfer of light occurs, by applying voltage (i.e.,giving a difference in index of refraction to the two waveguides) so asto give a difference to the propagation constants, the movement of lightis suppressed and switching is performed.

When it was attempted to apply this method to a multilayer typedirectional coupler using a conventional polymer electro-optic material,however, difficulties occurred. In a polymer electro-optic material, forexample, an electric field orientation treatment is performed to impartnonlinear optical characteristics, but the electrodes used for theelectric field orientation treatment are used as they are as theelectrodes for bringing out the electro-optic effect. In this case, thechange in the index of refraction due to the electro-optic effectbecomes substantially equal for all layers and it is not possible togive a difference in propagation constants to two waveguides by applyingvoltage. Further, even in the case of using the third-order nonlinearoptical effect or the thermo-optic effect, in a multilayer typedirectional coupler, it is difficult to change the index of refractionfor just one waveguide.

A secondary nonlinear optical material, however, is only realized by asubstance which does not have inverted symmetry, and therefore, thepolymer has to be subjected to a poling treatment to orient themolecules in one direction. Such molecularly oriented polymers, however,have the defects of a gradual weakening of the orientation and a smallernonlinear effect due to the heat motion of the molecular chains. Toprevent this relaxation of orientation, it is effective to use a polymerwith a high glass transition temperature. Use of a polyimide, which is apolymer with a high heat resistance, is being looked at. From thisviewpoint, Wu et al. of Lockheed have obtained a diffusion typenonlinear optical polymer using a polyimide as a host by mixingmolecules with a large nonlinear optical effect into polyamic acid andperforming polyimidization while performing a poling treatment (J. W. Wuet al., Appl. Phys. Lett. 58. 225 (1991)). This material does notexhibit attenuation of the nonlinear optical response (i.e.,electro-optic effect: EO effect) due to the relaxation of orientationeven in the face of heat treatment of 150° C. for 10 hours or more, butthe EO coefficient is a few pm/V (EO coefficient of LiNbO₃ is 30 pm/V),and therefore, the material is not practical. The EO coefficient issmall in this material probably because the concentration of the guestnonlinear optical molecules in the diffusion type material cannot bemade that large.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide a polymernonlinear material with a large nonlinear optical effect and a superiorheat resistance.

Another object of the present invention is to overcome theabove-mentioned problems in the prior art and to provide a directionaloptical coupler or directional coupling type optical switch, which canbe used even in a multilayer type waveguide having two or more waveguidelayers on a substrate.

Other objects and advantages of the present invention will be apparentfrom the following description.

In accordance with the present invention, there is provided a nonlinearoptical material comprising a polyimide obtained from a diamine and/or adiacid anhydride or dithioacid anhydride substituted at a portion havinga nonlinear optical effect.

As the diamine, preferably, a compound having the structure of theformula (I): ##STR1## wherein m₁ is an integer of zero or more,preferably from 1 to 3 and n₁ is, independently, an integer of zero ormore, preferably from 1 to 4, may be mentioned. As the diacid anhydride,preferably a compound having the structure of the formula (II): ##STR2##wherein m₂ is an integer of zero or more, preferably from 1 to 3 and n₂is, independently, an integer of zero or more, preferably from 1 to 4,may be mentioned.

As the dithioacid anhydride, preferably a compound having the structureof the formula (III): ##STR3## wherein m₃ is an integer of zero or more,preferably from 1 to 3 and n₃ is, independently, an integer of zero ormore, preferably from 1 to 4, may be mentioned.

In accordance with the present invention, there is also provided aprocess for producing a nonlinear optical material composed of apolyimide obtained by the steps of: causing a reaction between a diamineand a diacid anhydride to obtain a polyamic acid, applying the polyamicacid to a substrate to form a coating, then heating the coating, whileapplying an electric field thereto.

In accordance with the present invention, there is still furtherprovided a process for producing a nonlinear optical material composedof a polyimide obtained by causing a diamine and a diacid anhydride or adithioacid anhydride to sublimate, vapor deposit, and polymerize on asubstrate in a chamber to obtain a polyamic acid and heating thepolyamic acid, while applying an electric field.

In accordance with the present invention, there is still furtherprovided a process for producing a nonlinear optical material composedof a polyimide obtained by causing a diamine and a diacid anhydride or adithioacid anhydride to sublimate, vapor deposit, and polymerize on asubstrate in a chamber in the presence of an electric field to obtain apolyamic acid and then heating the polyamic acid, while applying anelectric field thereto.

In accordance with the present invention, there is still furtherprovided a polymeric nonlinear optical material of the formula (IV)obtained from molecules having a benzocyclobutene structure substitutedat the portion having a nonlinear optical effect: ##STR4## wherein Z isa portion having a nonlinear optical effect and n₄ is an integer of 2 ormore, preferably from 5 to 5000.

As the portion Z having the nonlinear optical effect in formula (IV),preferably mention may be made of a residual group having a bond at atleast one type of benzene ring selected from the azobenzene structure offormula (V), the stilbene structure of formula (VI), the tolan structureof formula (VII), and the diacetylene structure of formula (VIII).##STR5## wherein D is a donor group and A is an acceptor group.

In the above-mentioned formulas (V) to (VIII), preferably the said donorgroup D is --NR¹ R², wherein, R¹ and R² are, independently, a hydrogenatom or a straight chain or branched chain alkyl group having one to 4carbon atoms and said acceptor group A is a --NO₂, --CN, or dicyanovinylgroup.

In accordance with the present invention, there is further provided aprocess for producing a polymer nonlinear optical material of theformula (IV) comprising ring-opening polymerization of monomer moleculeshaving a benzocyclobutene structure of formula (IX): ##STR6## wherein Zis a portion having a nonlinear optical effect.

The polymeric nonlinear optical material mentioned above may be producedby causing the monomer molecules having the benzocyclobutene structureof formula (IX) to sublimate, vapor deposit, and polymerize on asubstrate in a chamber to form a coating.

In accordance with the present invention, there is still furtherprovided a directional coupling type optical switch which causes thereflection of the core portions or cladding portions of two waveguidesin proximity with each other to change by the electro-optic effect, thethird-order nonlinear optical effect, or the thermo-optic effect usingthe above-mentioned polymer nonlinear optical materials and causes thecoupling constant between the waveguides to change.

It should be noted that here, when the indexes of refraction of the coreor intermediate cladding portions of the two waveguides are made tochange by, for example, the electro-optic effect, the coupling constantκ between waveguides changes. When the coupling length π/2κ is halved bychanging the κ, that is, when κ is doubled, switching of the waveguidelight becomes possible. In actuality, it is difficult to realize enoughof a change of the index of refraction so that the κ doubles by, forexample, the electro-optic effect. Therefore, the device length of thedirectional coupler is made (2n+1) times the coupling length so thatmultiperiodic transfer will occur in the device (see FIG. 2). In such aconstruction, by making the coupling length n/(n+1) times by changingthe index of refraction of the core or intermediate cladding layer, thatis, by making κ (n+1)/n times, it is possible to perform opticalswitching. The magnitude of κ changes tremendously with widths of thecores and intermediate claddings of the two waveguides. In a multilayertype directional coupler, it is easy to control the core thickness orthe cladding thickness on the submicron order by, for example, spincoating or vapor deposition, and therefore, it is possible to optimizethe magnitude of κ and make the device length a practical length (e.g.,several cm or less). A directional coupling type optical switch, basedon the above principle, may be a planar type as well.

In accordance with the present invention, there is further provided adirectional coupler, for a multilayer type waveguide having two or morelayers of waveguides on a substrate, formed between two waveguides bymaking the thickness of the intermediate cladding layer small in thecoupling region and large in the noncoupling region.

The coupling region of the multilayer type directional coupler may beset by changing the thickness of the intermediate cladding layer betweenthe two waveguides. Therefore, the intermediate cladding layer is formedin two stages by vapor deposition of a dielectric or spin-coating,dip-coating, a doctor blade, or other wet process. At this time, it isalso possible to successively change the thickness of the intermediatecladding layer to prevent loss of light due to bends or curves in thewaveguides. Also, the order of the two stages of processes in theformation of the layer may be reversed. As the material for forming theintermediate cladding layer, use may be made of an inorganic substancesuch as SiO₂, an organic polymer obtained by, for example, vapordeposition and polymerization.

In accordance with the present invention, there is further provided adirectional coupler, for a multilayer type waveguide having two or morelayers of waveguides on a substrate, formed between two waveguides bymaking the index of refraction large in the coupling region and small inthe noncoupling region.

The coupling interval of the above-mentioned multilayer type directionalcoupler may be set by changing the index of refraction of theintermediate cladding layer between two waveguides. Toward this end, theintermediate cladding layer is formed in two stages by vapor depositionof a dielectric using two types of masks. Further, it is possible toform the intermediate cladding layer as a uniform film including aphotosensitive dyestuff, then irradiate light through a mask to changethe index of refraction between the coupling region and noncouplingregion. As the material and method of fabrication of the cladding layer,mention may be made of vapor deposition of an inorganic dielectric,spin-coating, dip-coating, or vapor deposition and polymerization of anorganic polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the description setforth below with reference to the accompanying drawings, wherein:

FIG. 1 is a view schematically showing the structure of a directionalcoupler;

FIGS. 2(a) and 2(b) are views showing the state of progression of alight beam in a directional coupler of the present invention, in whichthe index of refraction of the core or cladding portions has beenchanged;

FIG. 3 is a view showing the method of synthesis of the material diaminein Example 1;

FIG. 4 is a view showing specifically the method of synthesis (i.e., thefirst half) of the material diamine of Example 1;

FIG. 5 is a view showing specifically the method of synthesis (i.e., thesecond half) of the material diamine of Example 1;

FIG. 6 is an IR spectrum diagram of the diamine synthesized in Example1;

FIG. 7 is a view showing the routine of synthesis of the polyimide ofExample 1;

FIG. 8 is a view showing the structure of a multilayer waveguide ofExample 3;

FIG. 9 is a graph showing the relationship between the applied voltageand intensity of the emitted light in a multilayer waveguide of thestructure shown in FIG. 8 of Example 3; and

FIG. 10 is a graph showing the state of change of thickness on asubstrate in Example 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first polymeric nonlinear material according to the presentinvention is a side chain type polyimide nonlinear optical material,which incorporates nonlinear optical molecules into the side chain ofthe polyimide molecular chain. In this way, by causing coupling of apolyimide and nonlinear optical molecules, even when the concentrationof the nonlinear optical molecules is made large, there is noprecipitation of the molecules as crystals, and therefore, it ispossible to make the concentration of the nonlinear optical moleculeslarge and increase the nonlinear optical effect of the material.Further, in a side chain type polyimide, the motion of the molecules isrestricted, and therefore, a heat resistance even better than adiffusion type polyimide can be expected.

To prepare this side chain type polyimide nonlinear optical material, adiamine or diacid anhydride having a large nonlinear optical effect isnecessary, but such a substance currently is not available. Therefore,we developed a method of synthesis making an azo dye with a largenonlinear optical effect a diamine without impairing the nonlinearoptical characteristics.

According to the present invention, to obtain a thin film of apolyimide, there are, for example, the method of mixing a diamine and adiacid anhydride, dissolving the resultant polyamic acid in a solvent,then using spin-coating or another wet process to form a film, thenheating for removal of the solvent, dehydration and cyclization and themethod of forming a polyamic acid thin film from a diamine and a diacidanhydride or dithioacid anhydride by vapor polymerization, then heatingfor dehydration and cyclization.

The second polymeric nonlinear material according to the presentinvention is a nonlinear optical material using a polymer obtained bypolymerization of benzocyclobutene. In this way, it is considered that apolymer obtained by polymerization of benzocyclobutene can suppress thedeterioration of characteristics due to relaxation of the orientationsince it has a glass transition temperature of at least 200° C. and ahigh heat resistance.

The monomer serving as the raw material of the polymer is comprised of aportion for expressing the nonlinear optical effect and thebenzocyclobutene portion for the polymerization reaction. As thenonlinear optical portion, azobenzene, stilbene, tolan, diacetylene,etc., which can exhibit a great effect as a nonlinear optical portionare suitable.

The directional coupling type optical switch according to the presentinvention constitutes a directional coupler wherein multiperiodictransfer of light occurs due to the fabrication of, for example, twowaveguides or waveguide layers in close proximity to each other and canperform optical switching with small changes in the couplingcoefficient. Further, the portion causing the changes in the index ofrefraction may be constituted by a polymer given a nonlinear opticaleffect by an electric field orientation treatment (for example, an epoxypolymer bonded with a diacetylene compound (Extended Abstracts (The 52ndAutumn Neeting, 1991); The Japan Society of Applied Physics, 11a-T-1.)It should be noted that in the actual structure, electrodes are providedfor applying voltage for causing the electric field orientationtreatment and the changes in the index of refraction, but these may beprovided as a pair of parallel electrodes on one substrate or may beprovided as counter electrodes on a substrate and the waveguide layer.

The multilayer waveguide directional coupler according to the presentinvention may be produced by forming the intermediate cladding layer byvapor deposition of a dielectric or a wet process, forming a dielectricat the coupling regions to exactly the necessary thickness by vapordeposition or a wet process without a mask, then placing a mask openedat the noncoupling region in close contact with the vapor depositionsurface and further vapor depositing a dielectric so as to change thethickness of the intermediate cladding layer at the coupling region andnoncoupling region or else by forming the intermediate cladding layer byvapor deposition of a dielectric, then placing a mask opened at thecoupling region in close contact with the vapor deposition surface andvapor depositing a high refraction index dielectric to exactly thenecessary thickness and placing a mask opened at the noncoupling regionin close contact with the vapor deposition surface and vapor depositinga low refraction index dielectric to exactly the same thickness so as tochange the index of refraction of the intermediate cladding layer at thecoupling region and noncoupling region.

According to the present invention, it is possible to fabricate adirectional coupler by forming the intermediate cladding layer by vapordeposition of a dielectric or a wet process, vapor depositing or spincoating a dielectric to exactly the necessary thickness in the couplingregion without a mask, then placing a mask opened at the noncouplingregion parallel to the vapor deposition surface away from that vapordeposition surface and further vapor depositing a dielectric, therebysuccessively changing the thickness of the intermediate cladding layerin the coupling region and noncoupling region. It should be noted thatit is possible to use a mask opened at the noncoupling region to vapordeposit the dielectric, then form the dielectric by vapor deposition orthe wet process without a mask or alternatively to reverse the order ofthe same. In the above method, it is also possible to coat a resist onthe coupling regions, then form a dielectric on the entire surface byvapor deposition or a wet process, remove the resist to open thecoupling portion, then further form a dielectric on the entire surfaceby vapor deposition or a wet process or it is possible to form adielectric on the entire surface, then coat a resist on the noncouplingregion, and etch the coupling region so as to change the thickness atthe coupling portion and the noncoupling portion or to form a dielectricon the entire surface, then cover the noncoupling region with a mask anddry etch the coupling portion so as to change the thickness at thecoupling region and noncoupling region. As the vapor depositeddielectric, use may be made of a transparent inorganic substance suchas, for example, silicon oxide (SiO₂), silicon monoxide (SiO), aluminumoxide (Al₂ O₃), tungsten oxide (WO₃), calcium fluoride (CaF₂), or avapor deposited polymerized film of an organic substance (such as, forexample, a polyimide, polyamide, polyurea, polyazomethine, epoxypolymer, but use may be made of a polymer nonlinear optical material ofthe present invention).

According to the present invention, a mask opened at the noncouplingregion is used to vapor deposit a low refraction index dielectric, thena mask opened at the coupling region is used to vapor deposit a highrefraction index dielectric. This order may also be reversed.Alternatively, a mask opened at the noncoupling region may used to vapordeposit a low refraction index dielectric, then a high refraction indexdielectric may be vapor deposited or formed by spin coating or dipcoating without a mask. Further, a resist may be coated on the couplingregion, then a low refraction index dielectric may be vapor deposited orformed by spin coating or dip coating on the entire surface, the resistmay be removed to open up the coupling region, then a high refractionindex dielectric may be vapor deposited or formed by spin coating or dipcoating. It should be noted that it is possible to prepare a directionalcoupler designed to change the index of refraction of the intermediatelayer in the coupling region and noncoupling region by making theintermediate cladding layer a dielectric including photosensitivemolecules, forming it to the necessary thickness on the coupling regionand noncoupling region as a whole, then irradiating light through a maskopened at the noncoupling region so as to cause a reduction of the indexof refraction by the photo reaction of the photosensitive molecules inthe noncoupling region. In this case, it is also possible to fabricate adirectional coupler designed to change the index of refraction of theintermediate cladding layer in the coupling region and the noncouplingregion by making the intermediate cladding layer a dielectric, includingphotosensitive molecules, forming this on the coupling region and noncoupling region as a whole to the necessary thickness, then irradiatinglight through a mask opened at the coupling region, thereby causing theindex of refraction to increase by the photo reaction of thephotosensitive molecules in the coupling region. As the dielectric, itis possible to use the above-mentioned inorganic substance. For theintermediate cladding layer, use may be suitably made of theabove-mentioned polymer nonlinear material of the present invention.

According to the present invention, provision is made of a specificpolymer nonlinear material superior in both the nonlinear optical effectand heat resistance. Further, according to the present invention, bymaking the thickness of the intermediate cladding layer small or bymaking the index of refraction of the intermediate cladding layer large,it is possible to set the desired coupling length even in a multilayertype directional coupler.

EXAMPLES

The present invention will now be further illustrated by, but is by nomeans limited to, the following Examples.

Example 1

N,N-bis(4-aminophenyl)-4-(4-nitrophenylazo)aniline having a nonlinearoptical effect of the formula (X): ##STR7## was synthesized by the routeand procedure shown in FIGS. 3, 4, and 5. The IR spectrum of theresultant diamine is shown in FIG. 6.

Next, a polyimide was synthesized by the route shown in FIG. 7 from thediamine compound of the formula (X) obtained above and the compound ofthe formula (XI): ##STR8##

A 0.148 g (0.00031 mol) amount of the compound (X) and 0.067 g (0.00031mol) of the compound (XI) were dissolved in 1.5 g ofN,N-dimethylacetoamide and agitated at room temperature for 6 hours.After this, the resultant mixture was charged into methanol to causeprecipitation. This was filtered out and dried to obtain the polyamicacid. A 15 wt % N,N-dimethylacetoamide solution of this polyamic acidwas spin-coated at 1000 rpm and 30s on a glass substrate withtransparent ITO electrodes to form a film. This substrate was subjectedto corona charging (Note: wire voltage of 6 kV and wire-substratedistance of 10 mm) and at the same time raised from room temperature to230° C. at 4° C./min, then was heated at 230° C. for 1 hour and loweredto room temperature at a rate of 4° C./min to cause orientation of themolecules and polyimidization. Gold electrodes were vapor deposited onthis film, then the electro-optic coefficient (r) was measured by thereflection method (C. C. Teng and H. T. Man, Appl. Phys. Lett. 56, 1734(1990)), wherein r₃₃ =6 pm/V was obtained. The sample was heated and thechange in r over time was measured, whereupon it was learned that anonlinear optical material with a high heat resistance was obtained withno reduction seen in the r₃₃ even after heat treatment at 150° C. for 3hours.

Example 2

A xylene solution (30 wt %) of formula (XII) ##STR9## was spin-coated at1500 rpm and 30s on a glass substrate with transparent ITO electrodes toform a film. The substrate was subjected to corona charging (Note: wirevoltage of 6 kV and wire-substrate distance of 10 mm) and at the sametime raised from room temperature to 230° C. at a rate of 4° C./min,then was heated at 230° C. for 1 hour to cause polymerization, then waslowered to room temperature at a rate of 4° C./min to cause orientationof the molecules. Gold electrodes were vapor deposited on this film,then the electro-optic coefficient (r) was measured by the reflectionmethod, wherein r=3 pm/V was obtained. The sample was heated at 100° C.for 1 hour and the change in r over time was measured, whereupon it waslearned that a nonlinear optical material with a high heat resistancewas obtained with almost no reduction seen in the r.

Example 3

SiO₂ was EB vapor deposited to 2 μm on a heat oxidized silicon substratehaving A1 linear electrodes (width of 10 μm). On the top thereof,polyamic acid obtained by the reaction between the compound (X) and thecompound (XI) was spin-coated and the polyimide coating having athickness of 1.5 μm was applied by heating at 250° C. for 2 hours.Further, a multilayer waveguide of the structure shown in FIG. 8 wasprepared by EB vapor deposition of SiO₂, application of the polyimidecoating, and fabrication of A1 electrodes. Next, poling (100 MV/m, 250°C., lhr) was performed to impart, to the polyimide coating, anelectro-optic effect characteristic and a channel type waveguide wasformed by changing the index of refraction by the molecular orientation.The Si substrate was cleaved to take out a 15 mm channel type waveguideportion. Polarized semiconductor laser light (1.3 μm) perpendicular tothe film surface was fired to the waveguide of one end. At this time,when a voltage is applied to the electrodes, the intensity of the lightemitted from the two waveguides changes as shown in FIG. 9. In this way,optical switching by application of an electric field was performed inthe multilayer type directional coupler. Further, after the above samplewas heat treated at 150° C. for 2 hours, the above-mentioned opticalswitching test was repeated. As a result, the same result as shown inFIG. 9 was obtained. Thus, the heat resistance in the electro-opticeffect of the polyimide was clearly exhibited.

Example 4

A film of polyimide having a thickness of 1.5 μm was applied to the Siwafer with the heat oxidized film as a core layer by the spin-coating ofpolyamic acid obtained by the reaction between the compound (X) and thecompound (XI) was spin-coated and by heating at 250° C. for 2 hours,then a mask with just the coupling portion of the directional coupleropened was placed a distance of 1 mm from the wafer, SiO₂ was EB vapordeposited, then an SiO₂ film was EB deposited to 1 μm without a mask.The state of the change of the thickness at this time was measured by asteps, whereupon the result shown in FIG. 10 was obtained. On the topthereof, the same polyimide film as the lower layer was coated at athickness of 1.5 μm, then a SiO₂ film was vapor deposited as an upperbuffer layer, whereupon a multilayer type directional coupler with acontrolled coupling length was obtained.

Example 5

A film of polyimide having a thickness of 1.5 μm was applied to the Siwafer with the heat oxidized film as a core layer by the spin-coating ofpolyamic acid obtained by the reaction between the compound (X) and thecompound (XI) was spin-coated and by heating at 250° C. for 2 hours,then a mask with Just the coupling portion of the directional couplerclosed was placed into close contact on the wafer, SiO₂ (n=1.48) was EBvapor deposited to 1 μm, then a mask with only the coupling portion ofthe directional coupler opened was placed into close contact, and Al₂ O₃(n=1.63) was EB vapor deposited to 1 μm. On the top thereof, the samepolyimide film as the lower layer was coated at a thickness of 1.5 μm,then a SiO₂ film was vapor deposited as an upper buffer layer, whereupona multilayer type directional coupler where the coupling length wascontrolled so that the two core layers were coupled in the Al₂ O₃cladding portion where the index of refraction was large and were notcoupled in the SiO₂ cladding portion where the index of refraction waslarge was obtained.

As explained above, according to the present invention, provision ismade of a polymeric nonlinear optical material superior in both thenonlinear optical effect and heat resistance. Further, since the indexesof refraction of the core portions or cladding portions of two directlyconnected waveguides are changed to change the coupling constant betweenwaveguides, optical switching can be effectively carried out. Further,in a multilayer type waveguide, by changing the thickness or index ofrefraction of the intermediate cladding layer between the noncouplingregion and the coupling region, a directional coupler is constitutedbetween two waveguides, and therefore, it is possible to obtain adirectional coupler suitable for use in a multilayer type waveguide.

We claim:
 1. A nonlinear optical polymer material comprising: apolyimide composed of a diamine having a structure of the formula (I):##STR10## wherein m₁ is an integer of zero or more and n₁ is,independently, an integer of zero or more, covalently bonded with (ii) adiacid anhydride or dithioacid anhydride.
 2. A nonlinear optical polymermaterial comprising a polyimide composed of (i) a diamine covalently.bonded with (ii) at least one component selected from the groupconsisting of (a) diacid anhydrides having a structure of the formula(II): ##STR11## wherein m₂ is an integer of zero or more and n₂ is,independently, an integer of zero or more and (b) dithioacid anhydrideshaving a structure of the formula (III): ##STR12## wherein m₃ is aninteger of zero or more and n₃ is, independently, an integer of zero ormore.